The raw data collected during a measurement of time-resolved optical emission from an operating integrated circuit consists of individually detected photons, where for each photon coordinates x,y, and t (i.e., two spatial coordinates x,y representing the location of the occurrence in the sample plane plus a time coordinate t representing the time at which the photon was emitted) are required to be stored. This data is collected by repeatedly applying the same test input under constant conditions to a circuit and monitoring the location and time of photon emissions resulting from switching events in the circuit. Repetition is required because in most cases, and particularly in the case of integrated circuits fabricated from silicon, an individual switching event will result in a detected photon with a probability of much less than 1, and typically on the order of 10.sup.-6. A photon emission will be considered to have occurred for a given x,y,t if at least one of the iterations of processing the input signal resulted in the detection of a photon for that x,y,t triple. Typically, x, y and t can each be represented as an integer, where x and y might range from 0 to 1023, representing points on a grid defining the area of interest, while t might range from 0 to 8191, representing gradations of time over some period of observation. In order to store this data in histogram form, the array must contain 8 billion elements, or 16 gigabytes of memory, allocating 2 bytes per array element. Because of this large storage requirement, storage of the data in this form is usually impractical.
An alternative storage approach, called "list" storage, stores the data more efficiently when the number of detected photons is much less than the number of array elements in the histogram. In list storage, the data is stored as a long list of (x,y,t) coordinates, where each (x,y,t) triple represents the coordinates for a single detected photon. Using the numbers given above, and assuming that there are, for example, 20 million detected photons, the list file would only be 120 million bytes long, much smaller than the 16 gigabyte histogram file. List and histogram modes both contain the same information, just stored in different ways.